By applying information gained from earlier studies of the mechanisms of deamination reactions, we have devised a new method for the labelling of the active sites of enzymes. The substrate inhibitors used are nitrosoamides, triazenes, etc., which are patterned after the normal substrates, and which can produce highly active carbonium ions. The substrates are constructed so that the carbonium ions are produced only after the enzyme interacts with the substrate in the normal way. The carbonium ions are reactive enough to derivatize functional groups on the amino acids. Preliminary work suggests that they react principally with the oxygen atom of the amide. Hydrolysis of the imidate ester formed by this route should identify the amino acid that was "hit". Where the sequence of the enzyme is known, identification of which particular amino acid in the chain has been labelled should be easily determined by sequencing the chains; identification of the first 1-5 amino acids in the chain may well be sufficient to locate it in the overall sequence. Preliminary application of the above methods to chymotrypsin has been successful to date and further work is in progress. Extension of photoaffinity labelling by application of the above principles is also outlined.